XMPPCapabilities should update c element if present

Presence stanzas sent by XMPPStream's resendMyPresence may already
contain a c element. If a c element is present in the presence stanza
XMPPCapabilities should replace it rather than add an additional c
element.
XMPPvCardAvatarModule should update x element if present
Merge pull request #121 from andrewtj/atj-dedupe-presence-elements

XMPPCapabilities should update c element if present